1. Field of the Invention
The present invention relates to field of programmable filters and equalizers and, in particular, relates to programmable filters/equalizers for dual speed and density magnetic recording devices.
2. Statement of the Problem
The original IBM personal computer utilized a floppy controller having a transfer rate of 250 Kb/s (Kilobits per second) for transfer of data to a floppy disk. With the introduction by IBM of its Model AT, the transfer rate increased to 500 Kb/s. It is to be expected that future transfer rates will continue to increase, e.g., to 1000 Kb/s.
One "add-on" peripheral device in the marketplace is a tape backup drive which operates off the conventional floppy controller interface with the IBM (and others) personal computer. This tape drive serves as a backup for the data in the hard disk of the personal computer. Optimally, tape backup drives should operate at the highest transfer rate available in order to minimize the time for backing up the data. A problem exists in being capable of handling data backup in a single drive at the two conventional rates of 250 Kb/s and 500 Kb/s.
Presently, most tape backup manufacturers solve the dual transfer rate problem by designing two separate drives--one for the 250 Kb/s transfer rate and a second one for the 500 Kb/s transfer rate. This clearly increases inventory costs for both the drive manufacturer and the distributor of the drive manufacturer's products.
Other drive manufacturers optimize the design of their drive by designing the drive to function at the highest transfer rate and reading at both rates. Doing so, however, causes loss of performance in terms of error rate at the lower transfer rate. The amplitude of the signal coupled to the magnetic playback head is proportional to the rate of change of the magnetic flux with respect to time. A lower transfer rate equates to a decreased speed and, therefore, the signal output is decreased proportionately, causing an increase in the error rate at the lower transfer speed.
A need, therefore, exists to design a tape backup drive which can automatically sense between the available transfer rates and optimize the performance of the read channel to the fastest transfer rate. However, a need further exists that while operating at the lower transfer rate, any increase in error rate should be substantially eliminated. Hence, a single programmable filter which may be scaled down from the higher to the lower transfer rate wherein the error rate at the lower transfer rate does not increase is desired. A need exists to be able to modify the characteristics of the filter when changing tape speed so that the signal-to-noise ratio will not be reduced by the ratio of the change in tape speed. For example, decreasing the transfer rate from 1000 Kb/s to 500 Kb/s reduces the signal-to-noise ratio by 6 dB if the filter is unchanged. This results in a signal level decrease of 50% while the noise remain constant. A need exists for the characteristics of the filter to be scaled by one-half so as to maintain the same relationship between the cut-off frequency of the filter and the spectral content of the code so that the noise present would also be decreased by dB. A filter satisfying this need would allow a single backup tape drive to be optimized on either the PC or AT class of computers as required by the maximum available transfer rate.
3. Results of a Patentability Search
A patentability search was directed to the above problem which resulted in the following patents:
______________________________________ INVENTOR U.S. PAT. NO. ISSUE DATE ______________________________________ Gyi 4,314,288 2-2-82 Shah 4,371,901 2-1-83 Fischler et al. 4,894,734 1-16-90 ______________________________________
U.S. Pat. No. 4,314,288 sets forth an amplitude and phase equalizer which utilizes inductors, resistors, and capacitors to be selectively switched into or out from the frequency amplitude and phase equalizer so as to allow program control over the frequency thereof. The '288 patent finds application in single speed or multi-speed magnetic recording/playback devices.
U.S. Pat. No. 4,371,901 pertains to a programmable signal equalizer utilizing a plurality of active resistor-capacitor circuits arranged to be controlled by a ladder resistor network. The ladder circuits are selectively activated by digital word signals representative of the selective magnetic tape speed to produce a signal equalization operation at each of the selected tape speeds.
U.S. Pat. No. 4,894,734 sets forth a method and apparatus for constant density recording which provides a set of electronics that can be reconfigured upon selective switching to produce a desired frequency response. The selection is under control of a microprocessor.
Each of the above patented approaches provides its unique solution to the above stated problem. The '288 and '734 approaches, however, utilize expensive inductors. Inductors are costly and are sensitive to external fields. They are usually large (occupying valuable space on a printed circuit board) and generally have non-ideal performance. Hence, a further need exists, one not solved by the above two patents, in designing a programmable filter/equalizer which does not utilize inductors. The '901 patent does not utilize costly inductors, but utilizes expensive operational amplifiers. Hence, a further need exists to provide a programmable filter/equalizer which does not utilize expensive operational amplifiers.
Hence, while the above approaches provide various solutions to the above stated problem, the solutions are expensive to implement and exhibit non-ideal performance. Hence, a need exists for a low-cost programmable filter/equalizer which does not utilize inductors or operational amplifiers and which switches only resistors to obtain the different filter characteristics required to solve the above stated problem.
4. Solution to the Problem
The present invention is a programmable filter/equalizer for dual speed and density magnetic recording devices. The present invention utilizes a filter that is capable of scaling its magnitude and phase response over a 2:1 range as required by the dual transfer rates and a real densities of magnetic recording devices used in personal computers. The filter of the present invention is implemented utilizing three cascaded second order active filter stages and a first order passive filter stage which can be selectively switched. The same filter has the further advantages of reducing intersymbol interference (by pulse slimming) in the reading of data and it further delivers a differentiated output having the same delay as the filtered output.